1. 23 March 2005International TPC R&D Meeting Berkeley 1 Readout of a TPC using the Medipix2 CMOS pixel sensor (detection of single electrons on a direct pixel segmented anode) Jan Timmermans NIKHEF - Amsterdam

2. 23 March 2005International TPC R&D Meeting Berkeley 2 NIKHEFAuke-Pieter Colijn Arno Aarts Alessandro Fornaini Maximilien Chefdeville Harry van der Graaf Sipho van der Putten Peter Kluit Jan Timmermans Jan Visschers Saclay CEA DAPNIA Paul Colas Yannis Giomataris Arnaud Giganon Univ. Twente/Mesa+Jurriaan Schmitz CERN/Medipix ConstmErik Heijne Xavie Llopart Michael Campbell

3. 23 March 2005International TPC R&D Meeting Berkeley 3 Goals Gas multiplication GEM or Micromegas foil(s) Charge collection with granularity matching primary ionisation cluster spread Needs sufficiently low diffusion gas dE/dx using cluster counting? (  M. Hauschild) Proof of principle based on existing Medipix2 readout chip

4. 23 March 2005International TPC R&D Meeting Berkeley 4 Our GEM-equipped TPC We have constructed a small test TPC equipped with three GEM foils which can be read out by means of the MEDIPIX2 CMOS pixel sensor. MEDIPIX2 The GEM foils were obtained from the CERN/Sauli/GEM group; hole-to-hole distance (hexagonal geometry): 140 µm, hole diameter 85 µm, fiducial surface 100 mm x 100 mm, thickness 50 µm. The drift volume (vol. 100x100x100 mm 3 ) is surrounded by square wire loops, spaced 6.3 mm, put at decreasing potential. Three GEM foils are placed 7.4 mm behind the plane of the bottom wire loop; the distance between GEM foils is 1.6 mm. The anode plane, at ground potential, is 6.6 mm below the third GEM foil.

5. 23 March 2005International TPC R&D Meeting Berkeley 5 Drift Space GEM foils MediPix CMOS pixel sensor Brass spacer block Printed circuit board Aluminium base plate Medipix2: 256 x 256 pixels 55  m x 55  m area 14 x 14 mm 2 each pixel: low-noise preamp, discriminator, two threshold DAC, 13-bit counter, communication logic Drift length: 100 mm Distance between GEMs: 1.6/2.6 mm Distance bottom GEM/MEDIPIX: 6.6 mm

6. 23 March 2005International TPC R&D Meeting Berkeley 6 First events, recorded on March 29, 2003! Drift space irradiated with 55 Fe quanta Not immediately understood. Now we do: conversion source ~0.3 mm, defocussing GEM ~0.5 mm, diffusion in driftspace ~1.5 mm Ar/CH 4 90/10

7. 23 March 2005International TPC R&D Meeting Berkeley 7 exposed 0.01 sexposed 2 s exposed 0.1 s 90 Sr source; exposed 0.01 s Fiducial field: 14 x 14 mm 2 Collected ionisation in 14 x 14 x 100 mm 3 during exposure time Feb 9, 2004 Ar/Isobutane 95/5

8. 23 March 2005International TPC R&D Meeting Berkeley 8 MediPix2 pixel sensor Brass spacer block Printed circuit board Aluminum base plate Micromegas Cathode (drift) plane 55 Fe Baseplate Drift space: 15 mm With Paul Colas & Ioannis Giomataris: MediPix2 & Micromegas Very strong E-field above (CMOS) MediPix! S1S1

9. 23 March 2005International TPC R&D Meeting Berkeley 9 MediPix modified by MESA+, Univ. of Twente, The Netherlands Pixel Pitch: 55 x 55 μm 2 Bump Bond pad: 25 μm octagonal 75 % surface: passivation SiN New Pixel Pad: 45 x 45 μm 2 Insulating surface was 75 % Reduced to 20 %

10. 23 March 2005International TPC R&D Meeting Berkeley 10

11. 23 March 2005International TPC R&D Meeting Berkeley 11

12. 23 March 2005International TPC R&D Meeting Berkeley 12 We always knew, but never saw: the conversion of 55 Fe quanta in Ar gas No source, 1s 55 Fe, 1s 55 Fe, 10s Friday 13 (!) Feb 2004: signals from a 55 Fe source (220 e- per photon); 300  m x 500  m clouds as expected 14 mm The Medipix CMOS chip faces an electric field of 350 V/50 μm = 7 kV/mm !! Ar/Isobutane 95/5

13. 23 March 2005International TPC R&D Meeting Berkeley 13 New trial: March 30 - April 2, 2004 try to see single electrons from cosmic muons (MIPs) New Medipix New Micromegas (holds >500 V over 50  m in …..) He/Isobutane 80/20 (gas gain 10,000 – 20,000) Pixel preamp threshold: ~3000 e - Required gain: 5,000 – 10,000 ….. it works !

14. 23 March 2005International TPC R&D Meeting Berkeley 14

15. 23 March 2005International TPC R&D Meeting Berkeley 15 He/Isobutane 80/20 Modified MediPix 31 March 2004

16. 23 March 2005International TPC R&D Meeting Berkeley 16 He/Isobutane 80/20 Modified MediPix 31 March 2004

17. 23 March 2005International TPC R&D Meeting Berkeley 17 He/Isobutane 80/20 Non Modified MediPix Amaricium Source 1 April 2004

18. 23 March 2005International TPC R&D Meeting Berkeley 18 He/Isobutane 80/20 Modified MediPix 31 March 2004

19. 23 March 2005International TPC R&D Meeting Berkeley 19 He/Isobutane 80/20 Modified MediPix 31 March 2004

20. 23 March 2005International TPC R&D Meeting Berkeley 20 Example of a track reconstructed This track: #hits = 24 #clusters = 11 length (3d) = 16.8 mm 1.4 e - /mm; 0.65 cl./mm On average: 1.8 e - /mm; 0.5 cl./mm

21. 23 March 2005International TPC R&D Meeting Berkeley 21 Detection of single electrons using MediPix2/Micromegas assembly as Direct Pixel Segmented Anode NIM A540 (2005) 295 (physics/0409048) #pixels hit/mm track length = 1.83 #clusters/mm track length = 0.52 Single electron efficiency 0.9 Moire effects: mismatch in pixel and hole pitch: 60/(60-55) = 12 pixels repetition

22. 23 March 2005International TPC R&D Meeting Berkeley 22 Non Modified Modified Explanation for Moire effect:pitch Micromegas holes: 60 μm pitch MediPix pixels:55 μm Periodic position of hole w.r.t. pixel: repeats after 12 pixels!

23. Modified Non Modified InGrid: perfect alignment of pixels and grid holes! Small pad: small capacitance!

24. 23 March 2005International TPC R&D Meeting Berkeley 24 Recent work (with new Micromegas and new MediPix): Threshold studies (equalisation) Implementation external trigger (needed new Medipix software and interface box); it works! New data taking: much lower single electron efficiency (not yet understood) Kept MediPix chip alive for about 4 weeks!

25. 23 March 2005 International TPC R&D Meeting Berkeley 25 Integrate GEM/Micromegas and pixel sensor ‘GEM’‘Micromegas’ By ‘wafer post processing’ InGrid

26. 23 March 2005International TPC R&D Meeting Berkeley 26 Wafer dia.: 100 mm 30 fields with variety of pillar geometry First trials with pillars look OK Had some problems with ‘Micromegas’ grid (holes closed with very thin layer) First ‘good’ Ingrid in Feb. 2005 Reached 170V before breakdown (after that 80V with 4.6μA current) New design: -thicker grid layer (few microns) -larger spacing between fields -could be ready in few weeks

27. 23 March 2005International TPC R&D Meeting Berkeley 27 INGRID: some first trials Various pitches, shapes By ‘ wafer post processing’ at MESA+, Univ. of Twente pads pillars grid foils

28. 23 March 2005International TPC R&D Meeting Berkeley 28

29. 23 March 2005International TPC R&D Meeting Berkeley 29 with pillars with walls

30. 23 March 2005International TPC R&D Meeting Berkeley 30 TimePix1 Distribute clock to full 256x256 pixel matrix (100-160MHz) Enable counting by first hit after ‘shutter’ opens, until ‘shutter’ closes (common stop) Dynamic range 2 14 x 10 ns = 160 μs (for the time being) no zero-suppress to remain fully compatible with Medipix2 Shaping time ~200 ns, might be reduced for TimePix Extra static discharge protection for the front-end will be considered Keep same chip-size, pixel-size, readout protocol Can be done in 5-6 months (if done by MediPix2 designers)

31. 23 March 2005International TPC R&D Meeting Berkeley 31 Further Tests and Developments Investigate possible use of CMS (or Atlas) frontend pixel chip Ageing tests for…. GOSSIP: very thin gas/pixel detectors as Vertex Detector (LHC/ILC) (TIMEPIX2 with ~1 ns resolution) Chip tiling: large(r) detector surfaces (2x2, 2x4 chips) Through Si connectivity: avoiding bonding wires Fast readout technology (~5 Gb/s)

32. 23 March 2005International TPC R&D Meeting Berkeley 32 MediPix designed at CERN 4-chip tiling, serial readout and interface: NIKHEF 20 keV X-ray source at CERN Readout single chips 20 frames/sec New exp. software 70 frames/sec Next version chip several 10 3 frames/s

33. 23 March 2005International TPC R&D Meeting Berkeley 33 Backup slides

34. 23 March 2005International TPC R&D Meeting Berkeley 34 Top Drifter 5700 V Bottom Drifter 3473 V Top GEM 1 2813 V Bottom GEM 1 2462 V Top GEM 2 1876 V Bottom GEM 2 1524 V Top GEM 3 938 V Bottom GEM 3 586 V In the base plate of the chamber, a hole was cut out for the MEDIPIX2 chip: its pixel surface was flush with the (anode plane) base plate plane. The MEDIPIX2 chip contains 256 x 256 square pixels with pitch 55 µm x 55 µm giving a total fiducial sensitive area of 14.08 mm x 14.08 mm. Each pixel is equipped with a low-noise charge preamp, discriminator, two threshold DACs, a 13-bit counter and communication logic. Since a triggering system had not been implemented, we operated the MEDIPIX2 sensor by enabling the counters manually, and stop the counting after a pre-set time interval (0.1 - 10 s). After that, the counts of each pixel are read out. Drift length: 100 mm Distance between GEMs: 1.6/2.6 mm Distance bottom GEM/MEDIPIX: 6.6 mm

35. 23 March 2005International TPC R&D Meeting Berkeley 35 Eff = e -n/G n: threshold setting (#e-) G: Gas amplification Prob(n) = 1/G. e -n/G